As President Vladimir V. Putin of Russia made a rare acknowledgement of military challenges in the war, President Volodymyr Zelensky of Ukraine highlighted Russia’s struggles on Tuesday by making a bold visit to a frontline city that Moscow has failed to capture despite months of withering bombardment.

Zelensky’s visit — to Bakhmut, in the Donetsk region — was announced by his office hours after Putin acknowledged that conditions for Russia were “extremely complicated” in that region and three others in eastern and southern Ukraine that Moscow has illegally tried to annex.

In a transcript of a video address published early on Tuesday by the Kremlin, Putin also called on Russian security agencies to intensify their efforts “to put a firm stop to the activities of foreign special services and to promptly identify traitors, spies and diversionists”.

The video was released on a special holiday devoted to Russia’s security officials, and Putin used the occasion to highlight a “rapidly changing global situation and the emergence of new threats and challenges” to Russian security.

Russia claimed Ukraine’s Donetsk, Kherson, Luhansk and Zaporizhzhia regions in September, after orchestrated referendums, a move widely denounced by Ukraine and its Western allies as a violation of international law. But Russia has been experiencing setbacks on the battlefield over the course of the war.

Putin announced a “partial mobilisation” of troops in late September, which analysts said highlighted Russia’s major losses of professional soldiers.

In November, Russian forces ceded control of the city of Kherson, the only regional capital they had captured in nearly 10 months of the war.

In the video address, Putin acknowledged the “difficult tasks” facing the Russian security agencies working in the annexed areas, which he referred to as “Russia’s new regions”.

He framed the problem as one of ensuring the security, rights and freedoms of “Russian citizens”, as he claims the populations of those areas to be.

Putin charged the Russian authorities with strengthening their efforts, especially the border services of the FSB, the successor to the Soviet security agency, the KGB.

“Maximum composure, the concentration of forces is now required from counterintelligence agencies, including the military,” Putin said.